That got me wondering: How many capital–capital trips are there where taking
the train is faster than flying?

To find out, I used one of my favourite websites,
rome2rio, which gives you sensible route suggestions
and travel time estimates for most locations around the world, including travel
to and from the airport.

As far as I can see, there are 16 such routes around the world.
All my results are in this spreadsheet.

From

To

Δt/min

Comment

Sofia

Skopje

540

No direct flight

Belgrade

Priština

343

No direct flight

Ljubljana

Zagreb

154

No direct flight

Brussels

Luxemburg

149

No direct flight

Brussels

London

102

Paris

London

99

Brussels

Paris

88

Amsterdam

Brussels

66

Amsterdam

Paris

54

Paris

Luxemburg

48

Athens

Skopje

44

No direct flight

Bratislava

Budapest

41

Tallinn

Helsinki

40

Ferry!

Vienna

Budapest

34

Vilnius

Minsk

27

Bratislava

Vienna

No flight, obviously

As expected, all of these are in Europe. Nowhere else has the same combination
of outstanding infrastructure and density of capital cities.

One trip may be missing from this list: there is a ferry between Kinshasa
and Brazzaville across the Congo river, but I have no idea how this compares to
the stupidly short flight when taking into account all the overhead. Neither
Google nor rome2rio are much help here.

Let's get the obvious ones out of the way first: If you're going from London,
Amsterdam or Luxemburg to Paris or Brussels, you'll want to
take the train. The same naturally goes for any trip between your pick of
Vienna, Budapest and Bratislava.

Between Tallinn and Helsinki, the high-speed ferry has a bit of an edge
over the plane.

This list is particularly conspicuous through the near-absence of the Balkans
the Baltic sisters. The distance between Sarajevo and Belgrade is significantly
shorter than that between Brussels and Paris, but a train is nowhere to be found.
While there are trains from Riga to Tallinn and Vilnius, and the distance is
about the same as Paris–Brussels, this meandering Soviet inheritance has no way
to keep up with the shiny TGV, Thalys and Eurostar trains racing around France
and its neighbours.

Rail Baltica is a planned high-speed
rail link connecting the Baltic sisters to Poland, with potential onward
connections to Helsinki on one end and Berlin on the other.

While I was pretty much expecting these regions to do so poorly, I'm rather
disappointed in the mediocre rail links from Prague to Bratislava, Vienna
and Berlin.

Perhaps the biggest surprise in Europe is the connection from Athens to
Skopje: given the frosty relations between Macedonia and Greece and the mountainous
terrain, the fact that it takes eight hours to get from Athens to Skopje over
land makes sense; however, the closest place you can fly to directly is Thessaloniki,
which, overall, keeps the train surprisingly competitive.

The rest of the world

There are a few other places in Europe which are ‘near misses’, where flying
only just beats taking the train or bus; chief among them Paris–Berne (35 minutes faster)
and Tbilisi–Yerevan (52 min). The closest pair I could find outside of Europe,
apart from perhaps the aforementioned capitals of the Congos, is Buonos Aires
and its neighbour Montevideo: the four-hour ferry ride will get you to your
destination some two hours later than if you'd flown.

West Africa has plenty of countries with closish capitals. The bus between Lomé (Togo)
and Accra (Ghana) makes good time with only a two-and-a-half hour sacrifice with
respect to flying. Between Porto Novo (Benin) and Lagos, catching a car or microbus
of some sort might not be much slower than flying, but I have no idea how easy that is,
and I'm only counting ‘proper’ timetabled public transport here. That's
unfair to most of the world,
but anything else is practically impossible to estimate the travel time for, so
I'm rolling with it.

In the Caribbean, the ferry between Roseau (Dominica) and Castries (St Lucia)
make a strong showing considering that, well, it's a boat. I happen to know that
if you're going from Roseau to Fort-de-France in Martinique or Pointe-à-Pitre
in Guadeloupe, that same ferry beats flying easily, but those are departmental,
and not national capitals, so they don't count.

Some other pairs of capitals outside of Europe that are close in distance, but
not as well-connected, are Abu Dhabi–Muscat (7½ hour bus),
Port-au-Prince–Santo Domingo (6½ hour bus), and Kuala Lumpur–Singapore (8½ hour train,
6 hour bus).

In(coherent) conclusion: High-speed trains are great. More should be built.
Especially Rail Baltica. And direct connections from London to Amsterdam and
Frankfurt (Brexit be damned…).